The present invention relates to a feed length control device for use in consumable electrode welding operations in which a pulse-like welding current is employed.
FIG. 1 is a block diagram showing a conventional feed length control device used in consumable electrode welding operations.
In FIG. 1, reference numeral 1 designates a welding machine; 2, a torch for holding a wire electrode; and 4, a wire electrode feeding motor which rotates a pair of drive rollers 5 to feed the wire electrode 3. Further in FIG. 1, reference numeral 6 designates a torch moving mechanism for moving the torch 2 vertically; 7, a torch moving motor for operating the torch moving mechanism 6; 8, a metal base; 9, a current detector for detecting the welding current flowing between the wire electrode 3 and the metal base 8 supplied by the welding machine 1 and outputting a welding current detection signal generator; 10, a reference signal having a state determined according to the wire electrode feed length set in the welding operation; and 11, a comparator which compares the welding current detection signal outputted by the current detector 9 with the reference signal 10 to provide a deviation signal. Further in FIG. 1, reference numeral 12 designates a welding current setting unit for setting the peak current value, base current value, peak current time and base current time of a welding current; 13, a feed length control start instruction circuit which, in response to the deviation signal from the comparator and an output signal of the welding current setting unit 12 establishes the feed control timing; and 14, a motor drive circuit for amplifying the output signal of the circuit 13 to drive the torch moving motor 7.
In the feed length control device thus constructed, while the wire electrode 3 is being fed by the wire electrode feeding motor 4, a welding current is caused to flow between the wire electrode 3 and the base 8 so that arcing occurs between the wire electrode 3 and the base 8 to achieve the welding operation. As shown in FIG. 2, feed length control is carried out while the welding currrent is in the base current period. While the welding current is in the peak current period, feed length control is not carried out, but the welding current is varied to control the weld penetration or the amount of deposition.
In this case, the welding current set value is changed from a peak current set value to a base current set value instantaneously as shown in FIG. 2. However, the actual welding current set value cannot change from a peak current value to a base current value instantaneously. It has been found through measurement that it takes about 0.3 second for the welding current to change as described above. However, since feed length control starts at the time instant when the welding current set value is changed from the peak current set value to the base current set value as shown in FIG. 2, in practice, feed length control is started before the welding current changes from the peak current value to the base current value. Further, the reference signal from generator 10 for feed length control is made to coincide with the base current value. Therefore, disadvantageously the feed length is extremely long when feed length control starts.